1. Field of the Invention
The present invention relates generally to a multi-hop network, and in particular, to an apparatus and method for increasing retransmission efficiency by deciding which entity will retransmit when a Relay Station (RS) is available.
2. Description of the Related Art
Provisioning of services with diverse Quality of Service (QoS) levels at about 100 Mbps to users is an active study area for the future-generation 4th Generation (4G) communication system.
Particularly, active research is conducted to provide high-speed service by ensuring mobility and QoS to a Broadband Wireless Access (BWA) communication system such as Wireless Local Area Network (WLAN) and Wireless Metropolitan Area Network (WMAN). Such major examples are Institute of Electrical and Electronics Engineers (IEEE) 802.16d and IEEE 802.16e.
The IEEE 802.16d and IEEE 802.16e communication systems adopt Orthogonal Frequency Division Multiplexing/Orthogonal Frequency Division Multiple Access (OFDM/OFDMA) to physical channels of the WMAN system in order to support a broadband transmission network.
IEEE 802.16d considers only a single-cell structure with no regard to mobility of Subscriber Stations (SSs). In contrast, IEEE 802.16e supports the SS′ mobility to the IEEE 802.16d communication system. Hereinafter, a mobile SS will be referred to as an MS.
Since signaling is carried out between an MS and a fixed Base Station (BS) via a direct link, a highly reliable radio communication link can be established between them in the conventional IEEE 802.16e communication system.
However, due to the fixedness of BSs, a wireless network cannot be flexibly configured. As a result, the IEEE 802.16e communication system is not effective in efficiently providing communication services under a radio environment experiencing a fluctuating traffic distribution and a substantial change in the number of required calls.
These problems can be solved by applying a multi-hop relay data transmission scheme using RSs to conventional cellular wireless communication systems such as IEEE 802.16e.
The multi-hop relay wireless communication system can advantageously reconfigure a network rapidly according to a communication environmental change and enables efficient operation of the entire wireless network.
The multi-hop relay scheme uses a fixed RS, a mobile RS and a basic RS for relaying.
Another force behind the introduction of the multi-hop relay scheme to the cellular network is that a shadowing area caused by weak received signal strength is covered and installation of RSs mitigates the constraint of initial installation cost in an early stage with low service requirements.
As the cellular network adopts the multi-hop relay scheme using RSs, a network can be reconfigured rapidly according to the change of a communication environment and the entire wireless network can efficiently operate.
However, signals sent and received among a BS, an MS and an RS are vulnerable to distortion because they communicate on radio channels. Therefore, there exists a need for further increasing reliability among the BS, the MS and the RS in the multi-hop relay cellular network.
When the BS sends data to the MS wirelessly, the MS checks errors in the received data. If errors are detected, the MS notifies the BS of the error detection and the BS retransmits the data, thus increasing the reliability of wireless data communication.
FIG. 1 illustrates a conventional retransmission operation using an RS. Referring to FIG. 1, data (DATA 1 (111) and DATA 2 (112)) to be sent from a BS 110 to an MS 120 takes the form of MAC Header, Payload, and Error Check Code in a Media Access Control (MAC) layer.
The BS 110 broadcasts the data through an antenna within its cell area. If direction communication with the BS 110 is available to the MS 120, the MS 120 receives the data 111 and 112 directly. If the direct communication is unavailable, an RS 130 receives data 131 and 132 and forwards it to the MS 120. That is, the MS 120 receives the data from the BS 110 or via the RS 130.
If errors are detected in data 122 (DATA 2), the MS 120 requests retransmission of the data to the BS 110 or the RS 130. Correspondingly, the BS 110 or the RS 130 retransmits data 113 or 133 (DATA 2) to the MS 120.
When the MS receives the retransmission data without errors, the data transmission from the BS 110 or the RS 130 is completed.
Yet, the data retransmission to the MS is insufficient for fast retransmission, avoiding mutual interference among the BS 110, the RS 130 and the MS 120.
Accordingly, there is a need for an apparatus and method for deciding when, and by which entity will data be retransmitted according to, for example, channel status in data retransmission among the BS 110, the RS 130 and the MS 120.